1. Field of the Invention
The present invention relates to an alarm device for informing reduction of pneumatic pressure of tires in which when pneumatic pressure of tires of a vehicle is reduced by some reason while running or stopping of the vehicle this reduction is automatically sensed and the sensed signals are informed to drivers as an alarm information, particularly, a transmitter and receiver device for use in such an alarm device.
2. Description of the Invention
While running, particularly, high speed running of the vehicle, the reduction to less than the prescribed value of the pneumatic pressure of the tires makes the driver very dangerous so that the driver must know this reduction by some means. For this purpose various means relating to such device have been proposed. However, such device in the prior art has some defects and then the elimination of these defects is desired. Some of these defects pointed out in the prior arts are explained herinafter. FIG. 1 shows one embodiment of the relative relation between an oscillator of the prior device and a sensing coil for the reception of the incoming signals. The device comprises one set of a mechanism consisting of an oscillator 1, a power supply source 2, a sensing switch 3 which closes its contacts in case of lowering internal pressure of the tire to less than the prescribed value, and an electromagnetic radiation coil 4 for transmitting the output of the oscillator, and said mechanism is attached to the rotating wheel having tire. A sensing coil 5 is attached to the portion of a car body at the position nearly opposite to the coil 4. The output terminals 6 of the coil 5 are connected to a receiver 7 provided to the car body, respectively.
In this device when the internal pressure of any one of the tires is reduced to less than the prescribed value the sensing switch 3 is closed and thus a circuit including the power supply source 2 is closed so that the supply voltage is applied to the oscillator 1 and an alternating current thus generated flows through the coil 4. A part of an alternating magnetic field generated by the current through the coil 4 is passed through the coil 5 so that a voltage is induced in the coil 5 and thus an AC voltage is supplied to the receiver 7 from the output terminal 6 of the coil 5 resulting in the transmission and reception of the desired alarm information. The receiver 7 comprises, as shown in FIG. 2, receiver circuits Ra, Rb, Rc, and Rd, detection circuits Da, Db, Dc, and Dd and alarm circuits Aa, Ab, Ac, and Ad in channels 6a, 6b, 6c, and 6d of sensing coils. Alternatively, as shown in FIG. 3, normally closed switches Sa, Sb, Sc, and Sd are provided in the input channels of the system, respectively, and the outputs of these switches are OR-functionally connected to the receiver consisting of only one system of a receiver circuit R, a detection circuit D and an alarm circuit A. In the receiver constructed as shown in FIG. 2, when any one of the oscillators is operated the each corresponding alarm device of the system is operated so that this oscillator is separately identified. Moreover, in this receiver, the provision of many systems each consisting of a receiver, detector and alarm device becomes expensive.
In another known alarm device constructed as shown in FIG. 3 when either one of the oscillators is operated the receiver is operated through each switch formed by normally closed contacts. The identification of the oscillator related to the operation of the receiver is performed by sequentially making each switch OFF state. Then, the oscillator corresponding to the switch which stops the alarm at the OFF state may be identified. The defect of this device is that both the radiation coil 4 of the oscillator and the sensing coil 5 must be provided at the rear side of the wheel. In this case it is not preferable to provide the coil 4 near to the metal portion of the wheel because of large induction loss. On the contrary when the coil 4 is provided far from the metal portion of the wheel the attachment of the coil 4 is difficult because of no space between the wheel and the suspension portion of the axle. Therefore, the coil 4 is after all provided to the side wall of the tire at the rear side of the wheel. However, such provision makes the detachment of the tire from the wheel rim difficult when the punctured tire is changed. While the sensing coil 5 must be attached to the portion that relative distance between the sensing coil 5 and the radiation coil 4 is unchanged, i.e., to the axle housing forming a part of the suspension device of the vehicle. In addition, in order to lead the conductor wires to the receiver provided in the operator cabin the special machining to the completed car must be performed, and these elements must be attached every wheel. Moreover, in the receiver shown in FIG. 3 it is difficult to identify simultaneously operating oscillators.
The prior device above described utilizes an induction field in the propagation theory of electromagnetic wave in order to transmit the signals to the receiver from the oscillator and the radiator and the senser must be electromagnetically coupled or electrostatically coupled, i.e., nearly placed with each other so that the above described defects cannot be avoided.
On the contrary, when the radiation of the oscillator or the radiation field, i.e., the electromagnetic wave is utilized the relative distance between the radiator and the receiving antenna is large so that the intensity of the electromagnetic field at the receiving points falls off at the inverse square of its distance as compared with previous embodiment. While the SN ratio of desired signals and noises generated at the ignition system of the engine becomes worse so that the radiation power of the oscillator must be increased in order to improve this SN ratio but the capacity and the mass of the electric power supply source provided to the wheel are limited considering dynamic balancing and available space. While a mechanism of a slip ring and a brush or the like is required in order to supply the power from the car body rather than the wheel so that the attachment of this mechanism becomes very difficult and thus the increasing of the radiation power for improving the NS ratio cannot be easily realized in practice.